In the case of an electrical power assist steering system (referred to as EPAS), an electric motor actuator is provided for generating torque and applying the generated torque to a rack or linear steering member for steering of a vehicle. In the EPAS system, there is a mechanical connection between the steering wheel and the electric motor actuator wherein the torque generated by the electric motor actuator assist a driver's applied torque.
One concern with the electric motor actuator used to provide steering assist is the occurrence of electric motor actuator overheating. This is most likely to occur when the driver is engaged in a number of turns in high ambient temperatures or when the vehicle is powered-up, not in motion, and with the steering wheel held off center. To reduce the likelihood of overheating, designers have incorporated temperature measurement devices, such as a temperature sensor, which monitors the temperature of the control electronics of the electric motor actuator, or directly monitor the temperature of the electric motor actuator. The temperature sensor generates a signal indicative of the measured temperature which is utilized by a control algorithm to reduce the current applied to the electric motor actuator. A reduction in applied current thereby reduces the performance or level of assist steering torque, that in turn reduces the electric motor actuator temperature. The control algorithm will continue to reduce the applied current until the measured temperature falls within an acceptable temperature range.
However, in the above-mentioned prior art steering systems, the use of a temperature sensor increases the cost of the steering system, requires additional packaging considerations, and imposes a reliability factor. There is therefore a need for a steering system without a temperature sensor that will monitor the temperature of the electric motor actuator and consequently reduce the level of torque assist provided by the electric motor actuator when an unacceptable temperature condition is occurring.